Process for coating ferrous metal with aluminum



United States Patent OfiFice 3,027,259 Patented Mar. 27, 19fi2 This invention relates to improvements in the process for coating ferrous metal bases with aluminum by the hot dip process, and more particularly, to the hot dip aluminum coating process wherein the ferrous metal base is coated with an alkali metal compound and then subjected to heat treatment in the reducing atmosphere.

A principal object of the invention is to provide the formation of excellent corrosion resistant aluminum coating on ferrous metal free from voids, pin holes, blisters and yellow stain on the coated surface by the hot dip aluminum coating process.

In a conventional high speed hot dip aluminum coating process, such as the Sendzimir process, the aluminum coatings on a ferrous metal article obtained by this process have been found to be of inferior resistance to corrosion resulting from the tendency of formation of voids, pinholes and blisters, therefore it has been difficult to obtain a superior aluminum coating on iron and steel articles.

in order to overcome the above disadvantages, for example, U.S. Patent No. 2,437,919 proposes that a sodium, potassium or lithium aluminate layer be produced on the molten aluminum bath by charging metallic sodium, potassium or lithium into an open top container above the molten aluminum bath so that the heat of the latter evaporates sodium and allows its vapor to combine with impurities floating on the molten aluminum bath. However, the process disclosed in this patent has a disadvantage that it is far from safety in its operation.

No satisfactory aluminum coating on ferrous metal has been produced by the conventional high speed aluminum coating process utilizing a dry flux wherein a ferrous metal article to be coated is applied with a flux solution consisting of a compound selected from the group of borate, aluminum chloride, aluminum fluoride, alkali metal fluoride, alkali metal chloride, zinc chloride, ammonium chloride or their mixture, then dried, and thereafter coated with the molten aluminum.

Based on the results of our extensive research, however, we have discovered that a superior corrosion resistant aluminum coating free from voids, pinholes and blisters is attained by the present invention which comprises, briefly stated, applying an article to be treated with a thin layer of the aqueous solution consisting of an alkali metal compound or a mixture of two alkali metal compounds, drying the thus applied thin layer of the solution by heat, subjecting the thus treated article to heat treatment in a reducing atmosphere, and thereafter immersing the thus heat treated article in a bath of aluminum or consisting chiefly of aluminum. The invention is particularly effective for the high speed aluminum coating of steel strip.

More particularly, the process according to our invention is illustrated by the following flow sheet:

I Ferrous article Application of solution of alkali metal halide and/or hydroxide and/or azide to deposit 0.005 to 1.2 g./n1. of dry solute H L Drying by heating to above 400 C. for 5 to 30 seconds In Heating above 600 C. for ca. 0.2 to 5 minutes in reducing atmosphere exclusion of all contact with air Dipping for 2 to 15 seconds in plating bath of molten IV aluminum or aluminum alloy +interfacial alloy inhibitor V Withdrawal of aluminum plated product from bath In the field of known hot dip aluminum coating processes, a satisfactory aluminum coated article has been produced by the process which comprises applying to the article a known flux solution, and immediately coating it with aluminum, because the surface thereof has not been fully cleaned, and because it is dipped into the molten aluminum bath immediately after the heat treatment in the reducing atmosphere, provided that it is immersed in the coating metal bath for less than 15 seconds in either of the coating processes.

By the hot dip aluminum coating process of this invention in which the steel strip coated with the solution is heat treated in a reducing furnace, a satisfactory aluminum coating is obtained by a very short dipping time, because the surface of the steel is cleaned and wet by the molten metal bath.

-A preferred embodiment of the invention in connection with the aluminum coating of steel sheet is described in detail hereinbelow.

To a steel sheet to be coated with aluminum there is applied a solution of alkali metal compound (essentially, halide, hydroxide and azide) and an amount of from 0.005 to 1.2 grams per square meter of the above compound is caused to remain thereon after drying. The weight of the coating compound adhering to the surface of the steel is an important factor, and it depends on the concentration of the coating solution and also on the degree of squeezing after the application of the solution (step I in the above flow sheet).

When the amount of solution is excessive, the formation of stain, void and discoloration is inevitable, which results in an adverse efliect to the coating itself. It is to be understood that the solvent of the coating solution of this invention should not be confined to water, but any solvent which is able to dissolve the compound and vaporize or burn in a heating furnace may be used. Further, with a view to obtaining a uniform coating of the aqueous solution on the ferrous metal article, a surface active agent may be incorporated in the aqueous solution.

The application of the solution to the surface of the article may be effected either by immersing or spray, but it should be borne in mind that an amount of from 0.005 to 1.2 grams of the solute per square meter remains on the surface thereof after wiping off. The solution coating step is to provide the surface of an article coated with a uniform layer of the solution only so that a relatively short period of time may be sufiicient for dipping or spray.

The ferrous metal base thus coated with a uniform layer of the solution consisting of a compound selected from the group of alkali metal halide and alkali metal hydroxide is heated in a heating furnace of above 400 C. for a period of from 5 to 30 seconds so as to fully dry the solution thereon and also oxidize and burn the grease etc. present on the metal surface (step If in the flow sheet).

The temperature of the heating furnace depends upon the amount of the coating solution applied and of grease sticking to the metal surface and should be maintained so as to sufficiently dry and burn it. It is understood that the temperature should be high enough to give heat re quired to vaporize the solvent in case a metal sheet which has been degreased is used. The metal sheet leaving the heating furnace subsequently enters a furnace in which as reducing atmosphere is prevailing.

The object of this reducing furnace is to subject the metal surface to a reducing reaction as well as to a heat treatment so as to make it receptive to aluminum coating. The temperature range of the reducing furnace depends upon the speed of the processing line and the sheet gauge, and, generally speaking, is above the temperature of 600 C. The traveling time of the metal through the reducing furnace is between 0.2 and 5 minutes. It is necessary for the metal sheet base to be fully heat treated and reduced so as to make it receptive to aluminum coating. It is also necessary that the atmosphere of the heat treating furnace should be of a reducing nature (step III of the flow sheet).

The metal sheet leaving the heat treating furnace dips into the molten aluminum bath for a period of from 2 to seconds in such manner that the metal be absolutely out of contact with air.

The molten metal bath according to this invention consists of either pure aluminum or aluminum-base alloy to which is added an element selected from the group of silicon, lead, titanium and cerium in order to inhibit the development of an interfacial alloy in the coating layer, enhance the smoothness of the finished surface and improve heat resistance and corrosion resistance thereof.

It is understood that the molten metal bath will contain a very small amount of iron, because the molten aluminum or aluminum-base alloy bath dissolve iron at little, resulting from the iron material constituting the coating apparatus including a pot and a coating roll, and also from the ferrous metal to be coated with aluminum.

An aluminum or aluminum-base alloy coated steel sheet thus produced in accordance with the process of the invention is of a superior resistance to corrosion and also of a beautiful surface finish free from voids, pinholes, blisters and discoloration.

Various examples of the coating solution in accordance with the invention are described hereinbelow.

Based on the results of the aluminum coating process of the invention wherein various solutions are applied to the metal sheet, we have discovered that the solution consisting of a compound selected from the group of NaF, KF, NaOH, KOH and NaN has been effective for the prevention of the formation of particularly voids and blisters.

It is preferred that the amount of the compound adhering to the metal surface after drying be within the range from 0.025 to 1.2 grams per square meter, for example, 0.025 to 0.5 gram of NaF, 0.025 to 0.7 gram of KF, 0.1 to 1.2 grams of NaOH, 0.12 to 1.2 grams of KOH, and 0.15 to 1.2 grams of NaN respectively, per square meter.

If the amount of the compound adhering to the metal surface is Within the above range, the effect thereof is markedly, but, if it is less than that, it is not effective, or if it is more than that, it is disadvantageous, because discoloration tends to occur on the metal surface.

It has been also discovered that a solution containing of a mixture of the above compounds which is useful for the prevention of the formation of voids and blisters, for example, that of a mixture of NaF and KP, the amount of which adhering to the metal surface after drying is from 0.025 to 0.3 gram of NaF and from 0.025 to 0.5 gram of KP; a mixture of NaF and NaOI-I, the amount of which adhering to the metal surface after drying is from 0.025 to 0.3 gram of NaF and from 0.01 to 0.4 gram of NaOH; a mixture of KF and NaOH, the amount of which adhering to the metal surface after drying is from 0.025 to 0.5 gram of KF and from 0.01 to 0.4 gram of NaOI-I; a mixture of KF, NaF and NaOH, in an amount of from 0.025 to 0.5 gram of KF, 0.015 to 0.2 gram of NaF, 0.01 to 0.2 gram of NaOH; and a mixture of KF, NaF and KOH, in an amount of 0.025 to 0.5 gram of KF, 0.015 to 0.2 gram of NaF and 0.025 to 0.4 gram of KOH, all of said amounts being deposited respectively, per square meter of the metal surface.

Moreover, we have discovered that an addition of alkalimetal chloride and aluminum fluoride in a small quantity to the above solution is also effective. For example, they are as follows: a mixture of NaF, KCl and NaCl, the amount of which adhering to the metal surface after drying is from 0.025 to 0.5 gram of NaF, 0.01 to 0.14 gram of KCl, and 0.01 to 0.14 gram of NaCl; a mixture of KP, KCl and NaCl, the amount of which adhering to the metal surface after drying is from 0.025 to 0.7 gram of KF, 0.01 to 0.14 gram of KCl and 0.01 to 0.14 gram of NaCl, a mixture of KF and AlF in an amount of 0.025 to 0.7 gram of KF and 0.01 to 0.1 gram of A11 and a mixture of NaF and AIR, in an amount of 0.025 to 0.5 gram of NaF and 0.005 to 0.08 gram of AIF;,, each amount being deposited respectively, per square meter of metal surface.

Care should be taken not to exceed the amount of addition of either alkali metal chloride or aluminum fluoride to the coating solution, since an excessive addition may bring about discoloration on the coated surface of the metal article. When an aluminum coating is given to a metal surface after the application of the above solution to the metal surface and after the subsequent heat treatment in the reducing furnace as fully described hereinbefore, wetting of the metal surface with the molten aluminum is assured even in a very short period of time so that an aluminum layer free from void is obtained.

Water is generally used as a solvent for the solution, but a non-aqueous solution such as an alcoholic one may be used in order to accelerate either the drying of the solution or to improve the solubility of the solute.

Further, it has been found that the addition of asurface active agent in a small amount to the solution is more effective in bringing about a uniform application of the solution to the metal surface. More particularly, the addition of the surface active agent to the coating solution is indispensible for treating the metal sheet immediately issuing from the cold rolling step with a lubrieating oil thereon, otherwise a uniform coating of the solution on the metal surface will not be attained, which results in an inferior product.

We have conducted an aluminum coating process wherein a metal article is applied with a solution consisting of a known composition heretofore employed for aluminum coating, such as, aluminum chloride, zinc chloride, ammonium chloride, boric acid or borate, etc. in

various concentrations thereof, dried by heating, heat treated in the reducing furnace, and finally immersed in the molten aluminum bath out of contact with air for a period of from 2 to 20 seconds, but we have obtained no satisfactory product whatever.

Examples for carrying out the invention are described hereinbelow.

Example 1 To the surface of a steel sheet fully degreased, for example, by an electrolytic cleaning process there is applied an aqueous solution consisting of grams of NaF and 1 liter of water, the sheet is then dried at a temperature of above 400 C. in a heating furnace for 10 seconds; it then passes through a reducing atmosphere furnace heated to a temperature of above 600 C. in about 2 minutes, and finally it is dipped into a molten aluminum bath maintained at the temperature of 690 C. out of contact with air for a period of 7 seconds for aluminum coating.

Example 2 The same as Example 1, except that the metal is hot dipped in a bath consisting of 6% silicon and the balance of aluminum at a temperature of 660 C. for a period of 10 seconds.

Example 3 The same as either Example 1 or 2, except that an aqueous coating solution consisting of 35 grams of sodium fluoride and one liter of water is used.

Example 4 The same as either Example 1 or 2, except that an aqueous coating solution consisting of 20 grams of potassium fluoride and one liter of water is used.

Example 5 The same as either Example 1 or 2, except that an aqueous coating solution consisting of 400 grams of potassium fluoride and one liter of water is employed.

Example 6 The same as either Example 1 or 2, except that an aqueous coating solution consisting of 30 grams of sodium hydroxide and one liter of water is used.

Example 7 The same as the foregoing, except that there is used an aqueous coating solution consisting of 200 grams of sodium hydroxide and one liter of water.

Example 8 The same as the foregoing, except that there is used an aqueous coating solution of 40 grams of potassium hydroxide and one liter of water.

Example 9 The same as the foregoing, except that there is used an aqueous solution of 500 grams of potassium hydroxide and one liter of Water.

Example 10 The same as the foregoing, except that there is used an aqueous solution of 30 grams of NaN and one liter of water.

Example 11 The same as the foregoing, except that there is used an aqueous solution of a mixture of grams of KF and 8 grams of NaF with one liter of Water.

Example 12 an aqueous solution of a mixture of 10 grams of NaF and 5 grams of NaOH with one liter of water.

Example 14 The same as the foregoing, except that there is used an aqueous solution of a mixture of 30 grams of NaF and 15 grams of NaOH with one liter of water.

Example 15 The same as the foregoing, except that there is used an aqueous solution of a mixture of 20 grams of KF and 5 grams of NaOH with one liter of water.

Example 16 The same as the foregoing, except that there is used an aqueous solution of a mixture of grams of KF and 25 grams of NaOH with one liter of water.

Example 17 The same as the foregoing, except that there is used an aqueous solution of a mixture of 15 grams of KF, 8 grams of NaF and 5 grams of NaOH with one liter of water.

Example 18 The same as the foregoing, except that there is used an aqueous solution of a mixture of 45 grams of KF, 25 grams of NaF and 15 grams of NaOH with one liter of water.

Example 19 The same as the foregoing, except that there is used an aqueous solution of a mixture of 15 grams of KF, 8 grams of NaF and 10 grams of KOH with one liter of water.

Example 20 The same as the foregoing, except that there is used an aqueous solution of a mixture of 45 grams of KF, 25 grams of NaF and 30 grams of KOH with one liter of water.

Example 21 an aqueous solution of a mixture of 20 grams of KF and 3 grams of AIF;, with one liter of water.

Example 24 The same as the foregoing, except that there is used an aqueous solution of a mixture of 10 grams of NaF and 2 grams of AlF with one liter of water.

Example 25 The same as the foregoing, except that an alcoholic solution of 10 grams of NaOH in one liter of alcohol, C H OH is applied to the metal surface and dried at the temperature of 550 C. in a hot furnace for a period of 10 seconds.

Example 26 The same as the foregoing, except that 5 grams per liter of a surface active agent, Neogen EA No. 140, is added to the coating solutions of Examples 1 to 24, and the thus produced solution is applied to the metal surface, and then the thus applied surface is dried at a temperature of 700 C. in a hot furnace for a period of 15 seconds.

We claim:

1. In a hot dip aluminum coating process wherein a solution of an alkali metal compound selected from the group consisting of an alkali metal halide, an alkali metal hydroxide and an alkali metal azide is applied to the surface of a metal article to be coated with aluminum and dried prior to dipping into the molten aluminum bath, the improvement which comprises applying said solution to the surface of said article in such amount that from about 0.005 to 1.2 g./cm. of the solute thereof remain on said surface, subjecting said metal article to burning off grease adhering to the surface thereof, then to heat treatment at at least 600 C. in a reducing atmosphere thereby making said surface receptive to said coating, and then dipping said surface, with exclusion of air, into said molten aluminum bath.

2. In the hot dip aluminum coating of a ferrous metal article wherein the solution of a mixture of at least two alkali metal compounds selected from the group consisting of an alkali metal halide, an alkali metal hydroxide and an alkali metal azide is applied to the surface of said article and the surface is dried prior to dipping into the molten aluminum bath, the improvement which comprises applying the said solution to the surface of said article in such amount that from about 0.005 to 1.2 g./cm. of the solute thereof remain on said surface, subjecting said metal article to burning off the grease adhering to the surface thereof, then to heat treatment at at least 600" C. in a reducing atmosphere thereby making said surface receptive to said coating, and dipping said surface, with exclusion of air, into said molten aluminum bath.

3. Hot dip aluminum coating process which comprises applying a solution of an alkali metal compound selected from the group consisting of an alkali metal halide, an alkali metal hydroxide and an alkali metal azide, together with a surface active agent to the surface of a metal article in such amount that from about 0.005 to 1.2 g./cm. of the solute thereof remain on said surface, drying the thus coated surface, then burning off the grease adhering to said surface, then subjecting said surface to heat treatment at at least 600 C. in a reducing atmosphere thereby making said surface receptive to said aluminum coating, and dipping the thus treated article, with exclusion of air, into the molten aluminum bath.

4. In the hot dip aluminum coating of a ferrous metal article wherein the solution of a mixture of at least two alkali metal compounds selected from the group consistof an alkali metal halide, an alkali metal hydroxide and an alkali metal azide, together with a surface active agent is applied to the surface of said article and dried prior to dipping into the molten aluminum bath, the improvement which comprises applying the said solution to the surface of said article in such amount that from about 0.005 to 1.2 g./crn. of the solute thereof remain on said surface, subjecting said metal article to burning off the grease adhering to the surface thereof, then to heat treatment at at least 600 C. in a reducing atmosphere thereby making said surface receptive to said coating, and dipping said surface, with exclusion of air, into said molten aluminum bath.

5. In a hot dip aluminum coating process wherein a solution of an alkali metal compound selected from the group consisting of an alkali metal halide, an alkali metal hydroxide and an alkali metal azide is applied to the surface of a metal article to be coated with aluminumbase alloy and dried prior to dipping into a bath of the molen aluminum-base alloy, the improvement which comprises applying said solution to the surface of said article in such amount that from about 0.005 to 1.2 g./cm. of the solute thereof remain on said surface, subjecting said metal article to burning off grease adhering to the surface thereof, then to heating at at least 600" C. in a reducing atmosphere thereby making said surface receptive to said coatin and dipping said sur- 8 face, with exclusion of air, into said molten aluminumbase alloy bath.

6. Hot dip aluminum coating process which comprises I applying a solution of a compound selected from the group consisting of an alkali metal halide, an alkali metal hydroxide, an alkali metal azide, aluminum fluoride and a mixture thereof to the surface of a metal article to be coated With aluminum in such amount that from about 0.005 to 1.2 g./cm. of the solute thereof remain .on said surface, drying the thus coated article in a hot furnace heated to a temperature of above 400 C. for a period of 5 to 30 seconds, then burning off the grease adhering to said surface, then heat treating said article in a reduc ing furnace maintained at a temperature of above 600 C. for a period of from 0.2 to 5 minutes, thereby making said surface receptive to aluminum coating, and dipping said surface, with exclusion of air, into a bath of a member selected from the group consisting of molten aluminum and aluminum-base alloy for a period of at least 2 seconds.

7. Hot dip aluminum coating process which comprises applying an aqueous solution of sodium fluon'de to the surface of a metal article to be coated with aluminum in such amount that from about 0.005 to 1.2 g./cm. of the solute thereof remain on said surface, drying the thus coated article in a hot furnace heated to a temperature of above 400 C. for a period of from about 5 to 30 seconds, then burning off the grease adhering to said surface, then heat treating said article in a reducing furnace maintained at a temperature of at least 600 C. for a period of from about 0.2 to 5 minutes thereby making said surface receptive to coating with aluminum, and dipping said surface, with the exclusion of air, into a bath of a member selected from the group consisting of molten aluminum and aluminum-base alloy for a period of at least 2 seconds.

8. Hot dip aluminum coating process which comprises applying an aqueous solution of sodium fluoride together with a surface active agent to the surface of a metal article to be coated with aluminum in such amount that from about 0.005 to 1.2 g./cm. of the solute thereof re main on said surface, drying the thus coated article in a hot furnace heated to a temperature of above 400 C. for a period of from about 5 to 30 seconds, then burning off the grease adhering to said surface, then heat treating said article in a reducing furnace maintained at a temperature of at least 600 C. for a period of from about 0.2 to 5 minutes thereby making said surface receptive to coating with aluminum, and dipping said surface, with the exclusion of air, into a bath of a member selected from the group consisting of molten aluminum or aluminum-base alloy for a period of at least 2 seconds.

9. Hot dip aluminum coating process which comprises applying an aqueous solution of potassium fluoride to the surface of a metal article to be coated with aluminum in such amount that from about 0.005 to 1.2 g./cm. of the solute thereof remain on said surface, drying the thus coated article in a hot furnace heated to a temperature of above 400 C. for a period of from about 5 to 30 seconds, then burning off the grease adhering to said surface, then heat treating said article in a reducing furnace maintained at a temperature of at least 600 C. for a period of from about 0.2 to 5 minutes thereby making said surface receptive to coating with aluminum, and dipping said surface, with the exclusion of air, into a bath of a member selected from the group consisting of molten aluminum and aluminum-base alloy for a period of at least 2 seconds. 7

10. Hot dip aluminum coating process which comprises applying an aqueous solution of sodium hydroxide to the surface of a metal article to be coated with aluminum in such amount that from about 0.005 to 1.2 g./cm. of the solute thereof remain on said surface, drying the thus coated article in a hot furnace heated to a temperature of above 400 C. for a period of from about to 30 seconds, then burning oif the grease adhering to said surface, then heat treating said article in a reducing furnace maintained at a temperature of at least 600 C. for a period of from about 0.2 to 5 minutes thereby making said surface receptive to coating with aluminum, and dipping said surface, with the exclusion of air, into a bath of a member selected from the group consisting of molten aluminum and aluminum-base alloy for a period of at least 2 seconds.

11. Hot dip aluminum coating process which comprises applying an aqueous solution of potassium hydroxide to the surface of a metal article to be coated with aluminum in such amount that from about 0.005 to 1.2 g./cm. of the solute thereof remain on said surface, drying the thus coated article in a hot furnace heated to a temperature of above 400 C. for a period of from about 5 to 30 seconds, then burning off the grease adhering to said surface, then heat treating said article in a reducing furnace maintained at a temperature of at least 600 C. for a period of from about 0.2 to 5 minutes thereby making said surface receptive to coating with aluminum, and dipping said surface, with the exclusion of air, into a bath of a member selected from the group consisting of molten aluminum and aluminum-base alloy for a period of at least 2 seconds.

12. Hot dip aluminum coating process which comprises applying an aqueous solution of sodium azide to the surface of a metal article to be coated with aluminum in such amount that from about 0.005 to 1.2 g./cm. of the solute thereof remain on said surface, drying the thus coated article in a hot furnace heated to a temperature of above 400 C. for a period of from about 5 to 30 seconds, then burning oif the grease adhering to said surface, then heat treating said article in a reducing furnace maintained at a temperature of at least 600 C. for a period of from about 0.2 to 5 minutes thereby making said surface receptive to coating with aluminum, and dipping said surface, with the exclusion of air, into a bath of a member selected from the group consisting of molten aluminum and aluminum-base alloy for a period of at least 2 seconds.

13. Hot dip aluminum coating process which comprises applying an aqueous solution of a mixture of potassium fluoride and sodium fluoride to the surface of a metal anticle to be coated with aluminum in such amount that from about 0.005 to 1.2 g./cm. of the solute thereof remain on said surface, drying the thus coated article in a hot furnace heated to a temperature of above 400 C. for a period of from about 5 to 30 seconds, then burning off the grease adhering to said surface, then heat treating said article in a reducing furnace maintained at a temperature of at least 600 C. for a period of from about 0.2 to 5 minutes thereby making said surface receptive to coating with aluminum, and dipping said surface, with the exclusion of air, into a bath of a member selected from the group consisting of molten aluminum and aluminum-base alloy for a period of at least 2 seconds.

14. Hot dip aluminum coating process which comprises applying an aqueous solution of a mixture of sodium fluoride and sodium hydroxide to a surface of a metal article to be coated wtih aluminum in such amount that from about 0.005 to 1.2 g./cm.'-- of the solute thereof remain on said surface, drying the thus coated article in a hot furnace heated to a temperature of above 400 C. for a period of from about 5 to 30 seconds, then burning off the grease adhering to said surface, then heat treating said article in a reducing furnace maintained at a temperature of at least 600 C. for a period of from about 0.2 to 5 minutes thereby making said surface receptive to coating with aluminum, and dipping said surface, with the exclusion of air, into a bath of a member selected from the group consisting of molten aluminum and aluminum-base alloy for a period of at least 2 seconds.

15. Hot dip aluminum coating process which comprises applying an aqueous solution of a mixture of potassium fluoride and sodium hydroxide to a surface of a metal article to be coated with aluminum in such amount that from about 0.005 to 1.2 g./cm. of the solute thereof remain on said surface, drying the thus coated article in a hot furnace heated to a temperature of above'400 C. for a period of from about 5 to 30 seconds, then burning off the grease adhering to said surface, then heat treating said article in a reducing furnace maintained at a temperature of at least 600 C. for a period of from about 0.2 to 5 minutes thereby making said surface receptive to coating with aluminum, and dipping said surface, with the exclusion of air, into a bath of a member selected from the group consisting of molten aluminum and aluminum-base alloy for a period of at least 2 seconds.

16. Hot dip aluminum coating process which comprises applying an aqueous solution of a mixture of potassium fluoride, sodium fluoride and sodium hydroxide to a surface of a metal article to be coated with aluminum in such amount that from about 0.005 to 1.2 g./cm-. of the solute thereof remain on said surface, drying the thus coated article in a hot furnace heated to a temperature of above 400 C. for a period of from about 5 to 30 seconds, then burning off the grease adhering to said surface, then heat treating said article in a reducing furnace maintained at a temperature of at least 600 C. for a period of from about 0.2 to 5 minutes thereby making said surface receptive to coating with aluminum, and dipping said surface, with the exclusion of air, into a bath of a member selected from the group consisting of molten aluminum and aluminum-base alloy for a period of at least 2 seconds.

17. Hot dip aluminum coating process which comprises applying an aqueous solution of a mixture of potassium fluoride, sodium fluoride and potassium hydroxide to a surface of a metal article to be coated with aluminum in such amount that from about 0.005 to 1.2 g./cm. of the solute thereof remain on said surface, drying the thus coated article in a hot furnace heated to a temperature of about 400 C. for a period of from about 5 to 30 seconds, then burning off the grease adhering to said surface, then heat treating said article in a reducing furnace maintained at a temperature of at least 600 C. for a period of from about 0.2 to 5 minutes thereby making said surface receptive to coating with aluminum, and dipping said surface, with the exclusion of air, into a bath of a member selected from the group consisting of molten aluminum and aluminum-base alloy for a period of at least 2 seconds.

18. Hot dip aluminum coating process which comprises applying an aqueous solution of a mixture of sodium fluoride, potassium fluoride, potassium chloride and sodium chloride to a surface of a metal article to be coated with aluminum in such amount that from about 0.005 to 1.2 g./cm. of the solute thereof remain on said surface, drying the thus coated "article in a hot furnace heated to a temperature of above 400 C. for a period of from about 5 to 30 seconds, then burning off the grease adhering to said surface, then heat treating said article in a reducing furnace maintained at a temperature of at least 600 C. for a period of from about 0.2 to 5 minutes thereby making said surface receptive to coating with aluminum, and dipping said surface, with the ex clusion of air, into a bath of a member selected from the group consisting of molten aluminum and alumin base alloy for a period of at least 2 seconds.

19. Hot dip aluminum coating process which comprises applying an aqueous solution of a mixture of potassium fluoride, potassium chloride and sodium chloride to a surface of a metal article to be coated with aluminum in such amount that from about 0.005 to 1.2 g./cm. of the solute thereof remain on said surface, drying the thus coated article in a hot furnace heated to a temperature of above 400 C. for a period of from about 5 to 30 seconds, then burning off the grease adhering to said surface, then heat treating said article in a reducing furnace maintained at a temperature of at least 600 C. for a period of from about 0.2 to minutes thereby making said surface receptive to coating With aluminum, and dipping said surface, with the exclusion of air, into a bath of a member selected from the group consisting of molten aluminum and aluminum-base alloy for a period of at least 2 seconds.

20. Hot dip aluminum coating process which comprises applying an aqueous solution of a mixture of potassium fluoride and aluminum fluoride to a surface of a metal article to be coated With aluminum in such amount that from about 0.005 to 1.2 g./cm. of the solution thereof remain on said surface, drying the thug coated article in a hot furnace heated to a temperature of above 400 C. for a period of from about 5 to 30 seconds, then burning off the grease adhering to said surface, then heat treating said article in a reducing furnace maintained at a temperature of at least 600 C. for a period of from about 0.2 to 5 minutes thereby making said surface receptive to coating with aluminum, and dipping said surface, with the exclusion of air, into a bath of a member selected from the group consisting of molten aluminum and aluminum-base alloy for a period of at least 2 seconds.

21. Hot dip aluminum coating process which comprises applying an aqueous solution of a mixture of sodium fluoride and aluminum fluoride to a surface of a metal article to be coated with aluminum in such amount that from about 0.005 to 1.2 g./cm. of the solute thereof remain on said surface, drying the thus coated article in a hot furnace heated to a temperature of above 400 C. for a period of from about 5 to 30 seconds, then burning off the grease adhering to said surface, then heat treating said article in a reducing furnace maintained at a temperature of at lease 600 C. for a period of from about 0.2 to 5 minutes thereby making said surface receptive to coating With aluminum, and dipping said surface, with the exclusion of air, into a bath of a member selected from the group consisting of molten aluminum and aluminunrbase alloy for a period of at least 2 seconds.

22. Hot dip aluminum coating process which comprises applying an alcoholic solution of sodium hydroxide to a surface of a metal article to be coated With aluminum in such amount that from about 0.005 to 1.2 g./cm. of the solute thereof remain on said surface, drying the thus coated article in a hot furnace heated to a temperature of above 400 C. for a period of from about 5 to 30 seconds, then burning off the grease adhering to said surface, then heat treating said article in a reducing furnace maintained at a temperature of at least 600 C. for a period of from about 0.2 to 5 minutes thereby making said surface receptive to coating with aluminum, and dipping said surface, with the exclusion of air, into a bath of a member selected from the group consisting of molten aluminum and aluminum-base alloy for a period of at least 2 seconds.

References Cited in the file of this patent UNITED STATES PATENTS 2,197,622 Sendzimir Apr. 16, 1940 2,625,495 Cone et al. Jan. 13, 1953 2,824,020 Cook et a1. Feb. 18, 1958 2,955,958 Brown Oct. 11, 1960 

1. IN A HOT DIP ALUMINUM COATING WHEREIN A SOLUTION OF AN ALKALI METAL COMPOUND SELECTED FROM THE GROUP CONSISTING OF AN ALKALI METAL HALIDE, AN ALKALI METAL HYDROXIDE AND AN ALKALI METAL AZIDE IS APPLIED TO THE SURFACE OF A METAL ARTICLE TO BE COATED WITH ALUMINUM AND DRIED PRIOR TO DIPPING INTO THE MOLTEN ALUMINUM BATH, THE IMPROVEMENT WHICH COMPRISES APPLYING SAID SOLUTION TO THE SURFACE OF SAID ARTICLE IN SUCH AMOUNT THAT FROM ABOUT 0.005 TO 1.2 G./CM.2 OF THE SOLUTE THEREOF REMAIN ON SAID SURFACE, SUBJECTING SAID METAL ARTICLE TO BURNING OFF GREASE ADHERING TO THE SURFACE THEREOF, THEN TO HEAT TREATMENT AT LEAST 600* C. IN A REDUCING ATMOSPHERE THEREBY MAKING SAID SURFACE RECEPTIVE TO SAID COATING, AND THEN DRIPPING SAID SURFACE, WITH EXCLUSION OF AIR, INTO SAID MOLTEN ALUMINUM BATH. 